Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations

Purpose Fine roots and soil properties show distinct vertical patterns, reflecting their coupled responses to thinning and water-fertilizer management. This study aimed to elucidate soil-root interactions and provide insights for the sustainable management of plantations. Methods A split-plot design was established with three thinning intensities (no thinning, moderate, heavy) and three water-nitrogen treatments (control, irrigation, irrigation + nitrogen). Soil profiles (0–6 m) and fine roots were sampled to assess changes in soil moisture, nutrient dynamics, and fine root traits. Multivariate analyses were used to identify key regulatory drivers. Results Soil water content (SWC) peaked at 300–400 cm and was sensitive to management in the 20–500 cm layer. Thinning and irrigation increased SWC, whereas water-nitrogen input reduced it in mid-depth layers. Thinning enhanced nitrogen accumulation, while water-nitrogen input offset nitrogen loss but increased nitrate leaching risk. Fine root biomass density was highest in the 0–20 cm layer, with deeper layers remaining stable. Water-nitrogen addition increased specific root area, with SWC as the main determinant after thinning, and both phosphorus and SWC driving responses under fertilization. Conclusion Thinning improved water availability but constrained nutrients, while water-nitrogen input shifted fine roots toward an acquisitive strategy, highlighting management-specific soil-root interactions.